To build lighting systems for illumination and projection, there are significant advantages to being able to tailor the shape of the light source, since the shape of the light source and the optical components of the system provide the means to precisely shape the resulting light beam. The shape of the resulting light beam is an important aspect of the lighting system, especially in the creation of solid-state headlamps for the automotive industry, as disclosed in United States Published Patent Applications Nos. 2005/088853, entitled Vehicle Lamp, published Apr. 28, 2005 to Yatsuda et al; and 2005/041434, entitled Light Source and Vehicle Lamp, published Feb. 24, 2005 to Yasushi Yatsuda et al. The principle of operation is to construct an arrangement of light-source elements positioned in such a manner as to form an emission shape and a brightness distribution that can create a light distribution pattern when combined with suitable optics.
Unfortunately, conventional shaped light emitting devices must be constructed from a number of individual light emitting elements, such as LEDs, which typically cannot be constructed with an area greater than about four mm2 due to inherent limitations in compound semiconductor processing technologies, e.g. a lattice mismatch between substrate and active layers. Moreover, the individual light emitting elements typically cannot be positioned within five millimeters of each other, because of the need to provide physical mounting, optical coupling and electrical interconnection for each of the individual elements. Accordingly, the emissive shapes constructed do not provide a contiguous illuminated area, and have inherent limitations on the available brightness per unit area. Furthermore, the refinement or smoothness of the shape is limited by the granularity of the individual lighting elements, and the light emitting elements cannot be made smaller than a certain size because of the physical constraints in their mounting and interconnection.
Recent research into the nature of electrical conduction and light emission from nano-particles formed in wide bandgap semiconductor materials or insulating dielectrics has been conducted in an effort to increase the conductivity of the wide bandgap dielectric semiconductor materials, which exhibit very little conductivity, through the formation of nano-particles within the insulating material. With the application of a suitable electric field, current can be made to flow through the tunneling process, which can transfer energy efficiently from the applied electric field to the nano-particles and store that energy in the form of excitons through the impact ionization process in the silicon nano-particles. The excitons can radiatively recombine releasing a photon, whose energy is determined by the size of the nano-particles in the wider bandgap material or the nano-particles can transfer the energy to a rare earth dopant, which will emit a photon at a characteristic wavelength. A wide bandgap dielectric layer with nano-particles constitutes an optically active layer including a concentration of luminescent centers. Several materials can be used as the wide bandgap semiconductor or dielectric material including GaN, silicon nitride, and silicon dioxide. The luminescent centers can be formed from a wide variety and combination of compatible materials including silicon, carbon, germanium, and various rare earths.
For technical and economic reasons, Silicon Rich Silicon Oxide (SRSO) films are being developed for the purposes of studying the efficient generation of light from silicon based materials. The SRSO films consist of silicon dioxide in which there is excess silicon and possibly the incorporation of rare earths into the oxide. The amount of excess silicon will determine the electrical properties of the film, specifically the bulk conductivity and permittivity. With the excess silicon in the oxide, the film is annealed at a high temperature, which results in the excess silicon coalescing into tiny silicon nano-particles, e.g. nanocrystals, dispersed through a bulk oxide film host matrix. The size and distribution of the silicon nano-particles can be influenced by the excess silicon originally incorporated at deposition and the annealing conditions.
Optically active layers formed using semiconductor nano-particles embedded within a wider bandgap semiconductor or dielectric have been demonstrated in U.S. Pat. No. 7,081,664, entitled: “Doped Semiconductor Powder and Preparation Thereof”, issued Jul. 25, 2006 in the name of Hill; and U.S. Pat. No. 7,122,842, entitled Solid State White Light Emitter and Display Using Same, issued Oct. 17, 2006 to Hill; and United States Published Patent Applications Nos. 2004/151461, entitled: “Broadband Optical Pump Source for Optical Amplifiers, Planar Optical Amplifiers, Planar Optical Circuits and Planar Optical Lasers Fabricated Using Group IV Semiconductor Nanocrystals”, published Aug. 5, 2004 in the name of Hill; 2004/214362, entitled: “Doped Semiconductor Nanocrystal Layers and Preparation Thereof”, published Oct. 28, 2004 in the name of Hill et al; and 2004/252738, entitled: “Light Emitting Diodes and Planar Optical Lasers Using IV Semiconductor Nanocrystals”, published Dec. 16, 2004 in the name of Hill, which are incorporated herein by reference. The aforementioned references relate to different forms of the active semiconductor layer, and to the underlying physical principals of operation of the active semiconductor layers. Accordingly, no serious effort has been made to determine the structural requirements necessary to industrialize or provide practical solutions for manufacturing solid state light emitting devices including the active semiconductor layers.
With reference to FIG. 1, a conventional implementation of a practical light emitting device 1 including the above mentioned materials would consist of a starting conducting substrate 2, e.g. an N+ silicon substrate, on which an active layer 3 of a suitable thickness of a dielectric material, e.g. silicon dioxide, with a concentration of luminescent centers, e.g. rare earth oxides or nano-particles, deposited therein. The injection of electric current into the active layer 3 and the ability to view any light that might be generated within the active layer 3 will require a transparent conducting electrode, e.g. a transparent conductive oxide (TCO) layer 4 to be deposited on top of the active layer 3. Indium Tin Oxide, ITO, is currently the most widely used transparent conducting oxide in opto-electronic devices due to its excellent optical transmission and conductivity characteristics. ITO is a degenerately doped semiconductor with a bandgap of approximately 3.5 eV. Typical sheet resistances measured for the ITO range from as low as 10 Ω/sq to well over 100 Ω/sq. The conductivity is due the very high carrier concentrations found in this material. The work function of the ITO layer 4 is found to be between 4.5 eV and 4.8 eV depending on the deposition conditions. The work function of the N+ silicon substrate 2 is 4.05 eV. The difference in work functions between the ITO layer 4 and the silicon substrate 2 will result in an asymmetry in the electron current injection depending on which interface is biased as the cathode and injecting charge. The work function dominates the contact characteristics and is very important to the stable and reliable operation of any electro-luminescent device.
Subsequently, a metallization step is conducted forming ohmic contacts 5 and 6 onto the ITO layer 4 and the substrate 2, respectively, for injection of electric current. Application of high electric fields will be required for proper operation and the resulting current flow will consist of hot energetic carriers that can damage and change the electronic properties of the optical active layer 3 and any interfaces therewith.
As an example, the substrate 2 is a 0.001 Ω-cm n-type silicon substrate with an approximately 150 nm thick SRSO active layer 3, doped with a rare earth element for optical activity, deposited thereon. The transparent conducting electrode 4 is formed using a 300 nm layer of ITO. Finally metal contact layers 5 are formed using a TiN/Al stack to contact the front side ITO 4, and an Al layer 6 is used to contact the back side of the silicon wafer substrate 2.
At low electric fields in the SRSO active layer 3, there is no current flow and the structure behaves as a capacitor. With the application of an electric field larger than a characteristic threshold field, electrons can be injected into the SRSO active layer 3 from either the N+ substrate 2, via contact 6, or the ITO electrode 4, via contact 5, depending on their bias. Electrons residing in the potential wells due to the silicon nano-particles undergo thermal emission coupled with field induced barrier lowering to tunnel out of the nano-particle traps and into the conduction band of the host SiO2 matrix. Once in the conduction band of the host matrix, the electrons are accelerated by the applied electric field gaining kinetic energy with distance traveled. The distance between the silicon nano-particles will determine the total energy gain of the electrons per hop.
To produce green light at a wavelength of 545 nm, the SRSO active layer 3 may be doped with the rare earth dopant Erbium or Terbium. The energy associated with the emission of a 545 nm photon is approximately 2.3 eV. For current flow between the silicon nano-particles in the active layer 3 to be dominated by ballistic transport, the maximum spacing between the nano-particles should be <5 nm. For a 4 nm spacing, the minimum magnitude of the electric field is found to be approximately 6 Mv/cm, at which the conduction electrons can become quite hot and cause considerable damage to the oxide between the nano-particles through the generation of bulk oxide traps and at the interfaces between the silicon substrate 2 and active layer 3, and the active layer 3 and the ITO layer 4 through the creation of interface states. ITO may be susceptible to damage from high electric fields of approximately 1 MV/cm, which is believed may lead to the decomposition of In2O3 and SnO2. If the fields at the surface of the ITO are high enough, the indium and or tin ions can migrate with in the near surface region and concentrate at the active layer interface, this would cause a local reduction in the work function. The work function locally in this region would be reduced to approximately 4.4 eV and 4.2 eV for indium and tin, respectively, which would result in a significant increase in the electron injection characteristics of the ITO layer 4 and the formation of hot spots due to local current hogging potentially leading to device destruction.
The second effect that high electric field have on the device structure is the formation of trapped electronic states located in the band gap of the SiO2 region and interface states located at the active layer/silicon substrate. Generation of trap states in the SiO2 region will reduce the internal electric field and current conduction of the SRSO film requiring the application of higher electric fields to sustain a constant current flow. Positive charge trapping can also occur either through hole injection from the substrate or from impact ionization processes. For conduction electrons with energies >2 eV, traps are formed through the release of hydrogen decorated defects located at the anode. The hydrogen drifts under the applier field towards the cathode where it produces interface states capable of trapping electrons and limiting the current flow.
In the simple structure described above, planar breakdown of the active layer 3 at the edge of the light emitting device 1 will dominate and limit the electric field that can be applied thereto. All of these effects serve to modify, and in some instances increase, the internal electric field in the vicinity of the contact interfaces with the active layer 3, which will lead to an early breakdown and destruction of the light emitting device 1.
An object of the present invention is to overcome the shortcomings of the prior art by providing a light emitting structure in which field oxide regions are disposed below metal contacts to minimize edge related breakdown. Moreover, to overcome the problem of propagating breakdown and large area emitting apertures the total emitting area is subdivided into smaller area subpixel emitters that are laterally isolated from one another by the presence of a thick field oxide region.